The present invention relates to methods for detecting pressure differences and a devices for converting displacements in order to perform process control. More specifically, the present invention relates to a method for detecting pressure differences and a device for converting displacements which detects extremely small displacements in a diaphragm caused by pressure differences as differential changes in capacitance and converts these changes into a unified signal in order to perform process control.
Numerous difficulties are among the longstanding problems with using displacement converters to correct floating capacitances which have been addressed by the devices of the prior art. For example, in parallel flat plate types of sensors, use of known methods makes it possible to determine small diaphragm displacement values, and thus determine pressure differences.
However, conventional conversion characteristics must be measured and re-checked, repeatedly, in order to be confirmed. In practical terms, making high precision adjustments using this method requires numerous trial-and-error attempts. Thus, much time and effort is required to make adjustments.
Additionally, with conventional methods and devices, (such as parallel flat plate types of sensors) linearity is decreased because of changes in floating capacity caused by changes in temperature.
Finally, with regard to temperature characteristics for zero and span, corrections have to be made with combinations of temperature-sensitive resistors, thermistors and the like. However, precise corrections are not possible, requiring onerous trial-and-error attempts too numerous to be efficient.
According to known displacement converters of the parallel flat plate type of sensor type, (such as the present applicant's Japanese patent application no. 63-273120 entitled, DISPLACEMENT CONVERTER WITH IMPROVED LINEARITY) two additional capacitors were employed for compensation of floating capacitance.
The capacitances (or the equivalent capacitances from combinations with resistors and the like) C.sub.C1 and C.sub.C2 were adjusted so that C.sub.C1 =C.sub.S1 and C.sub.C1 =C.sub.S1. A voltage having a prescribed potential and prescribed frequency was applied to capacitances C1, C2, CC1 and CC2 in order to determine (C1-C.sub.C1) and (C2-C.sub.C2) from the charge current. By dividing the difference of these two by the sum, the following operation was performed: ( For this application "*" is used as a multiplication symbol "x") ##EQU1##
This equation makes it possible to determine very small displacement delta d of the diaphragm, and thus determine pressure difference P of the two sides of diaphragm 1.
However, as discussed above making high precision adjustments using this method requires numerous trial-and-error attempts, linearity is decreased because of changes in floating capacity caused by changes in temperature, and corrections have to be made with combinations of temperature-sensitive resistors, thermistors and the like. However, precise corrections have not been possible prior to the advent of the present invention.
In sum, among the prior art, hardware methods have been used in displacement converters to correct the floating capacitance contained in capacitances C1, C2 of the sensor capacitors. In the present invention, it is possible to perform linear, zero and span adjustments of a displacement converter easily and accurately. This is done by using capacitances C1(P), C2(P) measured beforehand for a plurality of measurement points with known pressure difference P, in order to calculate constants alpha and beta, which relate to the floating capacitance appearing at prescribed coefficient value f(P).
In contradistinction to known methods, these constants are used to determine function f based on the capacitance for the pressure difference, and the pressure difference is calculated and output. Thus, the floating capacitance is corrected using a software method.
The present invention performs the above corrections for predetermined temperature points beforehand, stores constants for each of these temperature points, measures the temperature as well as the sensor capacitor capacitance values when the pressure difference is measured, and uses the temperature-corrected constant to calculate and output the pressure difference. This provides a displacement converter having good linear, zero and span temperature properties.